1. Field of the Invention
This invention relates to an acetabular cup for a total hip prosthesis and total hip prostheses embodying such cups. More particularly, the invention relates to a cup having an outer shell designed to exhibit similar flexibility to the load bearing subchondral plate of the natural acetabulum and shaped to allow better load transfer characteristics.
2. Description of the Prior Art
In the mid 1970's total hip replacement designs were made which were classified as double cup arthroplasties. In these the femoral components had no intramedullary stem, but used a relatively thin part-spherical shell which was placed over the top of the femur from which a minimum of bone was removed. Inevitably, the bearing surface for such a design was large, approximating the normal anatomy. The acetabular component bearing surface also had to be large and there was little opportunity to remove a significant quantity of bone from the acetabulum. The acetabular cups for such designs therefore have thin wall thicknesses and are inherently flexible. At the period of development of these designs, almost all acetabular cups were cemented into position and these thin walled flexible acetabular cups flexed too much, thereby causing cracks which progressed around the bone's cement mantle or through it, leading ultimately to a loose acetabular implant. Furthermore, there tends to be elastic movement in the acetabulum, which causes distortion or deformation under load.
Typical examples of such total hips are shown in U.S. Pat. No. 4,123,806 and French Patent Application 2 361 861 (76 25215).
An analysis of the acetabulum in the human pelvis in the load bearing area shows that the load from the femur is transmitted and passes in a relatively direct line from that area up to the sacrum. The load is transmitted via a bar of trabeculae or a column of trabecular bone which is substantially straight so that in an x-ray of a human standing, it can be seen that there is a direct continuation of the medial compressive system of the proximal femur. It is therefore desirable that this area of the acetabulum is loaded and the remainder of the acetabulum should not have any load transmitted to the underlying bowl-shaped surface. A cup having an articular surface beyond the load bearing area, which area is substantially horse-shoe shaped, can only be justified if for some reason the articulation should be greater for the stability of the femoral head. This means that with a large headed hip prosthesis, the surface area of contact can be reduced to levels closer to that of a current conventional stem head cup arthroplasty.
A cup having a horse-shoe shaped opening formed in the shell is shown in U.S. Pat. No. 2,919,978 dated Nov. 3, 1959 and EPO Application 0 051 729 filed Sep. 19, 1981.
The present invention therefore is intended to provide a construction for an acetabular cup for acetabulum surface replacement which can be thin walled, and which has the ability to flex in harmony with bone movements without this leading to loosening of the implant (bone cements in use at present are not well suited to accommodate these movements, but it is possible that more flexible materials may be found in the future). At the same time, this cup is intended to provide a surface area of contact for a large headed hip prosthesis close to that of a current conventional stem head cup arthroplasty.
The present invention is intended to provide an acetabular cup which can be used for large hip head bearing surface diameters and which can also be used for smaller bearing surfaces of more conventional diameters, for example, 28 mm and 32 mm.